Which would you like me to go into details about? modern armor or early armor?

There are more things on the battlefield that can injure you than just bullets. Mostly debris and fragments from explosion. A helmet offers almost full protection from these. Combined with a good set of ballistic-goggles, the most vital parts of your head is protected. A helmet is not to be considered bulletproof, but I have seen quite a few examples of a bullet penetrating a helmet, changing direction, and bypassing the head. So a helmet is more than a psychological security-blanket.

Basically modern military armor consist of two things. Soft-body-armor (kevlar) and hard-body-armor (a solid plate made of various components). Soft-body-armor is only good for fragmentation and debris, and to some extend pistol fire. (which isn't too common on the battlefield)

Hard-body-armor is what will take force of a direct shot from a rifle, and is used in combination with soft-body armor. It comes in many classes. Some can take multiple shots while other shatter and crumple when hit. The later will usually be able to take a more powerful shot than the multiple-hit plates.

Even if a bullet should make it through the ballistic plate, there is a great change that the bullet is slowed down enough for the soft-body-armor to catch it.

The hard-body armor only covers the most vital organs (heart and lungs) This means you are still vulnerable from the sides, which have lead to a somewhat awkward looking combat stance that you might have seen on pictures or videos, where the soldier is standing with his full front toward his target and his rifle resting at the center of his chest. (as opposed to the earlier rifle stances where you more or less have your side toward your target).

One thing I'd like to add to the WW1 armor list, is the french cuirass. It was made in the early days of the war. Only a couple of thousands were made, but the extraordinary thing about them is that they weighted less than 3kg, meaning you could actually move around with them. They of course was not bulletproof at all, but could take fragmentation and battlefield debris. They did however have a drawback (as with other metal armor of the period), that if penetrated you wouldn't just have a bullet wound, but also have wounds from metal debris from the armor.